Transfer of autologous or heterologous genes into animal or human organisms with suitable vectors is emerging as a technique with immense potential to cure diseases with a genetic background or to prevent or cure infectious diseases. Several types of viral and non-viral vectors have been developed and tested in animals and in human subjects to deliver a gene/genes that are defective by mutations and therefore non-functional. Examples of such vectors include Adenovirus vectors, Herpes virus vectors, Retrovirus vectors, Lentivirus vectors and Adeno-associated vectors.
Vaccination has proven to be a highly effective and economical method to prevent a disease caused by infectious agents. Since the introduction of the Vaccinia virus as an attenuated vaccine against the smallpox virus (Variola), vaccines against a multitude of human pathogens have been developed and taken into routine use. Today small pox has been eradicated by vaccinations and the same is to be expected shortly for the poliovirus. Several childhood diseases, such as pertussis, diphtheria and tetanus, can be effectively prevented by vaccinations.
In general, the most successful viral vaccines are live avirulent mutants of the disease-causing viruses. The key to the success of this approach is the fact that a living virus targets the same organs, the same type and similar number of cells, and therefore, by multiplying in the recipient, elicits a long-lasting immune response without causing the disease or causing only a mild disease. In effect, a live attenuated vaccine produces a subclinical infection, the nature's own way of immunizing. As a result, a full immune response will be induced, including humoral, cellular and innate responses, providing a long lasting and sometimes a life-long immune protection against the pathogen.
Although live attenuated vaccines are most potent, they can cause harmful side effects. Thus, an attenuated viral vaccine can revert to a virulent strain or in cases where the attenuated virus is apathogenic in adults it can still cause a disease in infants or in disabled persons. This is true in the case of viruses causing chronic infections, such as Human Immunodeficiency Virus type 1 and 2. Vaccines composed of viral and bacterial proteins or immunogenic peptides are less likely to cause unwanted side effects but may not be as potent as the live vaccines. This is especially the case with vaccines against microbes causing chronic infections, such as certain viruses and intracellular bacteria.
The strength and type of immune response is, however, also dependent on how the viral proteins are processed and how they are presented to the immune system by antigen presenting cells (APCs), such as macrophages and dendritic cells. Protein and peptide antigens are taken up by APCs via endocytosis, processed to small immunogenic peptides through an endosomal pathway and presented to T-lymphocytes (T-cells) by MHC (major histocompatibility complex) class II antigens [in man HLAs (human leukocyte antigens) class II]. In contrast, proteins synthesized de novo in APCs or in possible target cells for an immune response, will be processed through a cytoplasmic pathway and presented to T-cells by MHC class I antigens (in man HLAs class I). In general, the presentation of immunogenic peptides through the class II pathway will lead to the activation of the helper/inducer T-cells, which in turn will lead to the activation of B-cells and to antibody response. In contrast, presentation through class I MHC favors the induction of cytotoxic T-lymphocytes (CTLs), which are capable of recognition and destruction of virally infected cells.
In early 1990's, a method to mimic the antigen processing and presentation that was normally achieved by live attenuated vaccines was introduced [Ulmer, J. B. et al Science 259 (1993) 1745-1749]. It was shown that an injection of eukaryotic expression vectors in the form of circular DNA into the muscle induced take-up of this DNA by the muscle cells (and probably others) and was able to induce the expression of the gene of interest, and to raise an immune response, especially a cellular immune response in the form of CTLs, to the protein encoded by the inserted gene. Since that observation. DNA immunization has become a standard method to induce immune responses to foreign proteins in experimental animals and human studies with several DNA vaccines are underway.
Generally, the DNA vectors used in these vaccine studies contain a cloning site for the gene of interest, a strong viral promoter, such as the immediate early promoter of the CMV virus, in order to drive the expression of the gene of interest, a polyadenylation region, and an antibiotic resistance gene and a bacterial replication origin for the propagation of the DNA vector (plasmid) in bacterial cells.
With the vectors described above it is possible to obtain a detectable level of expression of the gene of interest after administering the vector to experimental animals or to humans, either by a direct injection to muscle or to skin with a particle bombardment technique or by applying the vector in a solution directly to mucous membranes. However, the expression obtained by these vectors is short lived: the vectors tend to disappear from the transfected cells little by little and are not transferred to daughter cells in a dividing cell population. The short-term expression of the gene of interest and limited number of cells targeted are probably the major reasons, why only temporary immune responses are observed in subjects immunized with DNA vectors described above. Thus, for example, Boyer et al. observed only temporary immune responses to HIV-1 Env and Rev proteins in human subjects, who were immunized several times with a vector similar to the those described above [Boyer, J. D., J Infect Dis 181 (2000) 476-483].
There is a growing interest in developing novel products useful in gene therapy and DNA vaccination. For instance papilloma virus vectors carrying the expression cassette for the gene of interest have been suggested to be useful candidates.
To date more than 70 subtypes of human papilloma viruses (HPVs) and many different animal papilloma viruses have been identified [zur Hausen, H. and de Villiers E., Annu Rev Microbiol 48 (1994) 427-447; Bernard, H., et al., Curr Top Microbiol Immunol 186 (1994) 33-54]. All papilloma viruses share a similar genome organization and the positioning of all of the translational open reading frames (ORFs) is highly conserved.
Papilloma viruses infect squamous epithelial cells of skin or mucosa at different body sites and induce the formation of benign tumors, which in some cases can progress to malignancy. The papilloma virus genomes are replicated and maintained in the infected cells as multicopy nuclear plasmids. The replication, episomal maintenance, expression of the late genes and virus assembly are tightly coupled to the differentiation of the epithelial tissue: the papilloma virus DNA episomal replication takes place during the initial amplificational replication and the second, i.e. latent, and the third, i.e. vegetative, replications in the differentiating epithelium [Howley, P. M.; Papillomavirinae: the viruses and their replication. In Virology, Fields, B. C., Knipe, D. M., Howley, P. M., Eds., Lippincott-Raven Publishers, Philadelphia, USA, 1996, 2. Edition, p. 2045-2076].
Two viral factors encoded by the E1 and E2 open reading frames have been shown to be necessary and sufficient for the initiation of the DNA replication from the papilloma virus origin in the cells [Ustav, M. and Stenlund, A., EMBO J 10 (1991) 449-57; Ustav, M., et al., EMBO J 10 (1991) 4321-4329; Ustav, E., et al., Proc Natl Acad Sci USA 90 (1993) 898-902].
Functional origins for the initiation of the DNA replication have been defined for BPV1 [Ustav, M., et al., EMBO J 10 (1991) 4321-4329], HPV1a [Gopalakrishnan, V. and Khan, S., supra], HPV11 [Russell, J., Botchan, M., J Virol 69 (1995) 651-660], HPV18 [Sverdrup, F. and Khan, S., J Virol 69 (1995) 1319-1323:Sverdrup, F. and Khan, S., J Virol 68 (1994) 505-509] and many others. Characteristically, all these origin fragments have a high A/T content, and they contain several overlapping individual E1 protein recognition sequences, which together constitute the E1 binding site [Ustav, M., et al., EMBO J 10 (1991) 4321-4329; Holt, S., et al., J Virol 68 (1994) 1094-1102; Holt, S. and Wilson, V., J Virol 69 (1995) 6525-3652; Sedman, T., et al. J Virol 71 (1997) 2887-2996]. In addition, these functional origin fragments contain an E2 binding site, which is essential for the initiation of DNA replication in vivo in most cases (Ustav, E., et al., supra). The E2 protein facilitates the first step of the origin recognition by E1. After the initial binding of monomeric E1 to the origin the multimerization of E1 is initiated. This leads to the formation of the complex with the ori melting activity. It has been suggested that E2 has no influence on the following stages of the initiation of the DNA replication [Lusky, M., et al., Proc Natl Acad Sci USA 91 (1994) 8895-8899].
The BPV1 E2 ORF encodes three proteins that originate from selective promoter usage and alternative mRNA splicing [Lambert. P., et al., Annu Rev Genet 22 (1988) 235-258]. All these proteins can form homo- and heterodimers with each other and bind specifically to a 12 bp interrupted palindromic sequence 5′-ACCNNNNNNGGT-3′ [Androphy, E., et al., Nature 325 (1987) 70-739].
There are 17 E2 binding sites in the BPV1 genome and up to four sites in the HPV genomes, which play a crucial role in the initiation of viral DNA replication (Ustav, E., et al., supra) and in the regulation of viral gene expression (Howley, P. M., Papillomavirinae: the viruses and their replication, in Virology, Fields, B. C., Knipe, D. M., Howley, P. M., Eds., Philadelphia: Lippincott-Raven Publishers, 1996. 2. edition, p. 2045-2076). Structural and mutational analyses have revealed three distinct functional domains in the full size E2 protein. The N-terminal part (residues 1 to 210) is an activation domain for transcription and replication. It is followed by the unstructured hinge region (residues 211 to 324) and the carboxy-terminal DNA binding-dimerization domain (residues 325 to 410) [Dostatni, N., et al., EMBO J 7 (1988) 3807-3816; Haugen, T., et al. EMBO J 7 (1988) 4245-4253; McBride, A., et al., EMBO J 7 (1988) 533-539; McBride, A., et al., Proc Natl Acad Sci USA 86 (1989) 510-514]. On the basis of X-ray crystallographical data, the DNA binding-dimerization domain of E2 has a structure of a dyad-symmetric eight-stranded antiparallel beta barrel, made up of two identical “half-barrel” subunits [Hegde, R., et al., Nature 359 (1992) 505-512; Hegde, R., J Nucl Med 36(6 Suppl) (1995) 25S-27S]. The functional elements of the transactivation domain of E2 have a very high structural integrity as confirmed by mutational analysis [Abroi, A., et al., J Virol 70 (1996) 6169-6179; Brokaw, J., et al., J Virol 71 (1996) 23-29; Grossel, M., et al., J Virol 70 (1996) 7264-7269; Ferguson, M. and Botchan, M., J Virol 70 (1996) 4193-4199] and by X-ray crystallography [Harris, S., and Botchan, M. R., Science 284 (1999) 1673-1677 and Antson, A. et al., Nature 403 (2000) 805-809]. In addition, X-ray crystallography shows that the N-terminal domain of the E2 protein forms a dimeric structure, where Arg 37 has an important function in dimer formation (Antson, A., et al., supra).
As has been described previously, bovine papillomavirus type 1 E2 protein in trans and its multiple binding sites in cis are both necessary and sufficient for the chromatin attachment of the episomal genetic elements. The phenomenon is suggested to provide a mechanism for partitioning viral genome during viral infection in the dividing cells [Ilves, I., et al., J Virol. 73 (1999) 4404-4412].
None of the papilloma vectors or other vectors disclosed so far fulfills the criteria and requirements set forth for an optimal vaccine, which are the same for DNA vaccines and for conventional vaccines. (It should be noted that these requirements are preferred but not necessary for use as a vaccine.) First, an optimal vaccine must produce protective immunity with minimal adverse effects. Thus the vaccine should be devoid of components, which are toxic and/or cause symptoms of the disease to the recipient. Second, an optimal vaccine must induce a pathogen-specific immune response, i.e. it must elicit a strong and measurable immune response to the desired pathogen without causing an immune response to other components of the vaccine. These two requirements imply that a vector to be used as a DNA vaccine should optimally only express the desired gene(s) and optimally should not replicate in the host or contain any sequences homologous with those of the recipient, since nucleotide sequences that are homologous between the vector and the host's genome may effect the integration of the vector into the host's genome. Third, an optimal vaccine must induce a right type of immune response; i.e. it must raise both humoral and cellular immune responses in order to act on the intracellular and extracellular pathogen. Finally, an optimal vaccine must be stable, i.e. it must retain its potency for a sufficiently long time in the body to raise the immune response in a vaccine formulation for use in various demanding circumstances during storage and preparation. Additionally, vaccines should be of reasonable price. Further, the route and the method of inoculation are important considerations for optimizing a DNA immunization.
When developing a DNA vaccine the stability of the expression of the desired gene is sometimes a major problem. Thus, the maintenance function or the persistence of the vector in the recipient cell has been focused on in the prior art, however, often at the cost of the safety. For example, Ohe, Y., et al.][Hum Gene Ther 6(3) (1995) 325-333] disclose a papilloma virus vector capable of stable, high-level gene expression, which is suggested for use in gene therapy. Transforming early genes E5, E6, and E7 have been deleted from said vector, but it still contains nucleotide sequences encoding other papilloma viral genes, such as the E1 and E2 genes, which are involved in the replication of the virus. Thus, the vector produces several other papilloma proteins, which may elicit undesired immune responses and which induce a risk of the vector's integration in the recipient. Also, the vector is replicable, since it contains the E1 gene. Additionally, it is large in size and therefore subject to bacterial modification during preparation.
International Patent Application PCT/EE96/00004 (WO 97/24451) discloses vectors capable of a long-term maintenance in a host cell and methods using such vectors for obtaining long-term production of a gene product of interest in a mammalian host cell, which expresses E1 and E2. These vectors contain a minimal origin of replication of a papilloma virus (MO), a Minichromosome Maintenance Element (MME) of a papilloma virus and a gene encoding said gene product, the MO and MME consisting of a DNA sequence different from the natural papilloma virus sequence, and in some embodiments the E1 gene. Additionally, vectors containing an MME consisting essentially of ten E2 binding sites are disclosed in some examples. These vectors require the presence of the E1 protein either in the host or in the vector for the expression. This imparts the replication function to the vectors. These vectors also express the E1 protein in addition to the gene of interest and the E2 protein and contain sequences, such as rabbit β-globin sequences, which are partially homologous to human sequences causing a serious risk of integration to human genome, which reduces the potential of these vectors as DNA vaccines. Additionally, the vectors are unstable due to their size (ca 15 kb): at the preparation stage in a bacterial cell, the bacterial replication machinery tends to modify the vector by random slicing of the vector, which leads to unsatisfactory expression products including products totally lacking the gene of interest.
International Patent Application PCT/EE96/00004 (WO 97/24451) further discloses that E1 and E2 are the only viral proteins necessary for the episomal long-term replication of the vectors. Additionally, the maintenance function of the BPV1 genome is associated with the presence of minimal ori (MO), which is stated to be necessary, although not sufficient, for the long-term persistence or the stable maintenance of the vectors the cells. In addition, the cis-elements, i.e. the Minichromosome Maintenance Elements of the BPV1, are stated to be required for the stable replication of BPV1. In particular, multimeric E2 binding sites (E2BS) are stated to be necessary for the stable maintenance of the vectors.
There is a clear need for improved novel vectors, which would be useful as DNA vaccines.
An object of the invention is therefore to provide novel vectors, which are capable of a long-term maintenance in a large and increasing number of different cells of the host's body and thereby capable of providing a stable expression of the desired antigen(s).
Another object of the invention is to provide novel vectors, which are maintained for a long period of time in the cells that originally received the vector and transferred it to the daughter cells after mitotic cell division.
Yet another object of the invention is to provide novel vectors, which express in addition to the gene or genes of interest preferably only a gene necessary for a long-term maintenance in the recipient cells and thus are devoid of components that are toxic or cause symptoms of the disease to the recipient.
A further object of the invention is to provide novel vectors, which mimic attenuated live viral vaccines, especially in their function of multiplying in the body, without inducing any considerable signs of disease and without expressing undesired proteins, which may induce adverse reactions in a host injected with the DNA vaccine.
Still a further object of the invention is to provide novel vectors, which do not replicate in the recipient.
Still another object of the invention is to provide novel vectors, which induce both humoral and cellular immune responses when used as DNA vaccines.
Yet another object of the invention is to provide novel vectors, which are suitable for a large-scale production in bacterial cell.
Yet another object of the invention is to provide novel vectors, which are not host specific and thus enable the production in various bacterial cells.
An additional object of the invention is to provide novel vectors, which are useful as carrier vectors for a gene or genes of interest,
A further object of the invention is to provide novel vectors, which are useful in gene therapy and as gene therapeutic agents and for the production of macromolecular drugs in vivo.